The use of a linear integrating cavity as part of an illuminator for a photographic film scanner is well known. A basic integrating cavity has a cylindrical cavity comprising an interior white diffusing surface of high reflectivity, a light entry port and a light exiting slot. Light from an external source is input into the cavity through the entry port. The light is scattered throughout the cavity and exits through the slot to provide uniform diffuse light for exposure to the photograhic film in the scanner. Commonly assigned U.S. Pat. No. 5,241,459, the disclosure of which is incorporated herein by reference, describes such a cavity in an illuminator system that is also provided with a feedback port for the sampling of light within the cavity. This light sample is conveyed via a fiber optic cable to a detector and automatic control circuit. The output of the automatic control circuit is coupled back to the light source to control the light source so as to avoid fluctuation in the intensity of the light beam entering the cavity.
In such illuminator system using feedback control for the light source, it has been found that in some cases the intensity of the light exiting the slot varies even though the feedback controls are working properly. The problem appears to occur primarily when the input light source is spatially distributed, as in the case of a xenon arc lamp, for example, in which the spatially distributed arc moves in a manner commonly referred to as "arc wander". There is therefore a need for feedback control in a light integrating cavity type of illuminator system utilizing a spatially distributed light source that is immune to effects of arc wander in the light source.